1) Field of the Invention
The present invention relates to the manufacture of composite structures and, in particular, to the securing of composite structures such as honeycomb sandwich structures to a mandrel to prevent distortion during manufacture.
2) Description of Related Art
Composite structures are manufactured for use in a variety of structural applications where the structures provide high stiffness-to-weight and strength-to-weight ratios. For example, a honeycomb core sandwich panel has composite laminate skins that are co-cured with adhesives to opposite sides of a lightweight honeycomb core that can be formed of paper, metal, and the like. Such structures are described in U.S. Pat. Nos. 5,284,702; 4,622,091; and 4,353,947, each of which is incorporated herein in its entirety by reference.
One problem encountered during the manufacture of composite structures relates to the flow of resin from the laminate skin to the core of the structure. Resin is typically provided on or in the skins, and the resin is cured in an autoclave during the co-cure operation. For example, the skins can be formed of a pre-impregnated material, i.e., a prepreg, such as a sheet of a matrix material with a resin disposed therein. As described in U.S. Pat. No. 5,604,010 which is incorporated herein in its entirety by reference, the resin from the skins can flow into the core of the structure during manufacture, thereby reducing the amount of resin in the laminates, and reducing the strength of the laminates. The resin that flows into the core does not add significantly to the strength of the structure. Thus, a heavier panel must be used to achieve a specific desired strength.
Scrim-supported barrier films are described in U.S. Pat. No. 5,604,010 between the fiber-reinforced resin composite laminates and the honeycomb core for preventing the flow of the resin into the core. In addition, U.S. Pat. Nos. 5,685,940 and 5,895,699 describe the use of tiedown plies to reduce slipping of the barrier film relative to the core during curing, and both patents are incorporated herein in their entirety by reference. The edges of the tiedown plies can be adhered to one another using a film adhesive with a relatively low cure temperature so that the film adhesive melts and cures at a lower temperature than the resin in the laminates. Thus, the film adhesive can be used to bond the tiedown plies together before the autoclave pressure and temperature are increased during curing of the laminates, thereby retaining the structure in a desired configuration during curing and reducing the likelihood of the core being crushed between the laminates.
The edges of the tiedown plies are also taped to a mandrel on which the composite structure is disposed to further retain the composite structure to its desired configuration. However, the use of the tape adds to the time and cost of the manufacturing process. Further, in order to sufficiently secure the tiedown plies, each tiedown ply extends beyond the desired size of the structure, with successively higher tiedown plies in a stacked structure extending outwardly beyond each of the lower plies so that each ply can be taped to the mandrel. This requires additional material for the tiedown plies, and requires additional space on the mandrel for securing each of the tiedown plies.
Alternatively, grit strips, also referred to as “grip strips,” can be provided on the mandrel to increase the friction between the mandrel and the structure and retain the structure in the desired configuration. The grit strips can be formed of a gritty layer, similar to sandpaper, that is secured on the mandrel, or the grit strips can be formed of metal strips that are roughed by forming a plurality of perforations therethrough. However, the grit strips also add to the manufacturing cost. Further, the grit strips can be difficult to clean after use, requiring additional time for cleaning or replacement.
Thus, there exists a need for an improved mandrel and method for retaining structural members during manufacture. The mandrel and method should be applicable to composite structures such as composite honeycomb structures, and should retain the structure to prevent changes in the configuration of the structure, for example, to prevent core crush. Further, the improvement should preferably not require that the mandrel be significantly larger than the composite structure.